The well-known Rankine cycle and its various derivatives (e.g., Kalina cycles or organic Rankine cycles) are used in approximately 80% of worldwide electricity production. Rankine cycles can be used with a wide array of energy sources, including fossil fuels, nuclear, geothermal, solar, and bio-fuels. The demand for electric energy is constantly increasing, and therefore so to is the use of Rankine cycles.
In a basic Rankine cycle, a working fluid such as water is pumped into a boiler where it is heated by an external heat source to produce a vapor. The high pressure vapor is routed to a turbine where it expands to rotate the turbine and produce output power. The pressure and temperature of the vapor decrease as it expands through the turbine. The vapor then exits the turbine and enters a condenser where it is condensed back into a saturated liquid. This liquid “feed” is then pumped back to the boiler and the cycle repeats.
Most Rankine cycles include one or more additional features to increase efficiency. For example, in a regenerative Rankine cycle, the feed returning from the condenser is preheated before being fed to the boiler. The feed is preheated in a feedwater heater (FWH) using a bleed stream of hot vapor from elsewhere in the system, e.g., from the turbine. While the bleed introduces slight losses in turbine output, the gains from preheating the feed produce a net increase in efficiency.
Rankine cycles have been around for a long time, and their design and operation have been extensively studied to the point that their operation is considered near optimum. Even a small increase in efficiency of these cycles, especially under practical and realistic operating conditions, can result in major economic and environmental advantages. Accordingly, a need exists for Rankine cycles having improved efficiency.